1. Field of the Invention
The present invention relates generally to a device for determining the concentration of NOx (oxides of nitrogen) contained in a measurement gas, and more particularly to an NOx concentration sensor suitable for determining or measuring the concentration of Nox contained in exhaust gases or emissions produced by automotive internal combustion engines or various industrial furnaces, in order to control the combustion or burning condition of the engines or furnaces.
2. Discussion of the Prior Art
As a device for detecting or determining the oxygen concentration of gases, there is known an oxygen sensor which utilizes an oxygen-ion conductive solid electrolyte such as zirconia ceramics. An example of this type of oxygen sensor is disclosed in laid-open Publication No. 63-10781 (published March 9, 1988) of examined Japanese Patent Application. This oxygen sensor includes an electrochemical sensing cell and an electrochemical pumping cell each of which has an electrode exposed to an internal space which communicates, through suitable diffusion-resistance means, with the external space in which a gas to be measured (measurement gas) exists. The diffusion-resistance means provides a suitable diffusion resistance to the measurement gas. A controlled pumping current is applied to the pumpimg cell, based on the electromotive force induced by the sensing cell according to the principal of an oxygen concentration cell, so that the pumping cell operates to control the oxygen concentration of the atmosphere within the internal space to a predetermined level. The oxygen concentration of the measurement gas is determined by the pumping current when the oxygen concentration in the internal space is maintained at the predetermined level. Namely, the electromotive force V.sub.s induced by the sensing cell is compared with a reference voltage V.sub.r, and the pumping current I.sub.p applied to the pumping cell is controlled in a feedback manner so that the electromotive force V.sub.s coincides with the reference voltage V.sub.r.